"Women who jog while expecting are not more likely to have babies born prematurely or of a low birthweight, according to the largest study of its kind," reports the Mail Online.

This follows a new UK study investigating whether participating in recreational running during pregnancy increased the risk of premature birth and low birthweight.

The researchers questioned more than 1,000 women who took part in runs arranged by Parkrun, a non-profit organisation that regularly puts on 5km runs across the UK.

They found that most babies were born at full term, with no difference in rates of premature birth (between 6% and 7% in both groups) or low birthweight. About 5% more women in the runner group needed an assisted vaginal delivery, such as the use of forceps during labour. However, the study's design means it's not possible to say running directly caused this – it could be a chance difference.

While the findings are largely reassuring for women who want to continue to run while pregnant, it shouldn't be concluded that any level of vigorous or strenuous exercise is safe during pregnancy. Regular exercise is important during pregnancy, but you should be guided by your prior level of fitness and what feels comfortable.

Guidelines currently recommend that continuing with a moderate level of exercise – 30 mins daily, for example – is safe for pregnant women. High-impact sports are not usually recommended, and it's best to consult your midwife or GP if you're considering an intensive running programme.

Where did the story come from?

The study was carried out by researchers from King's College London, with no sources of funding reported. It was published in the peer-reviewed British Medical Journal (BMJ) Open Sport and Exercise Medicine on an open-access basis, so it's free to read online.

The Mail Online's coverage was generally accurate, but it would have been useful if it had highlighted that the study was carried out among regular runners. Pregnant women clearly have differing levels of prior health and fitness and, while continuing with a previous level of activity is generally suitable, high-impact sports are not usually recommended during pregnancy.

In other words: if you've never taken part in a 5km run before, pregnancy may not be the best time to start.

What kind of research was this?

This was a cohort study investigating whether running during pregnancy affected when the baby was born and its birthweight. A birth prior to the 37th week of pregnancy is considered premature.

More than 2 million people in the UK are reported to run every week on a regular basis, and 60% of marathon runners are known to be women. However, running is a high-impact activity that is not usually recommended in pregnancy. Previous studies have raised concerns of harmful effects on the baby, such as restricting blood flow to the placenta and affecting growth.

Observational studies are a useful way to study the relationship between running during pregnancy and subsequent birth outcomes. However, it isn't possible to directly attribute any specific outcome to running, as you can't fully rule out the multitude of other health and lifestyle factors that could be having an influence.

What did the research involve?

The researchers recruited 1,293 women who took part in Parkrun's 5km runs between November 2014 and January 2015. They sent an online questionnaire to women who had previously been pregnant and collected information on birth outcomes and running habits during their first pregnancy.

They first analysed the responses according to whether the women continued to run during pregnancy or not and then compared women who never ran at all with women who stopped during different trimesters (first, second or third) of pregnancy. Mean weekly running distance during pregnancy – not just time spent on their Parkrun – was split into 3 groups: 1 to 5km, 6 to 12km and more than 12km.

The researchers looked for associations between running and the following birth outcomes:

birthweight

assisted vaginal delivery (forceps or ventouse suction)

prematurity (before 30 weeks, before 34 weeks and before 37 weeks)

What were the basic results?

Out of all the participants:

45% did not run during pregnancy

15% stopped during their first trimester

25% ran into their second trimester

16% ran into their third trimester

There was no significant difference in average pregnancy duration at delivery between women who continued running and those who stopped: 279.6 days compared with 279.0 respectively (mean difference 0.6 days, confidence interval [CI] 1.3 to 2.4). That is, most babies were born at full term of around 40 weeks.

There was no significant difference between runners and non-runners in prematurity rates (6% among runners and 7% among non-runners) or the proportion of babies classed as having a low birthweight.

"Randomised prospective analysis is necessary to further explore these findings."

Conclusion

This study of a large group of regular runners was useful to see whether running during pregnancy may be linked with the risk of prematurity and low birthweight. The researchers found no difference between those who continued running during pregnancy and those who stopped.

However, it's important to note that the participants in this study were all regular runners. Women who consistently take part in recreational running tend to have higher fitness levels and may lead healthier lifestyles in terms of diet, smoking and alcohol consumption. They may also be less likely to have any pre-existing medical conditions, such as diabetes. Therefore, these findings cannot easily be applied to the general population of pregnant women.

Furthermore, because the study was observational, it can't prove cause and effect. For example, although running was linked with higher rates of assisted delivery, we can't say this was directly due to running.

The retrospective design of the study – asking women to recall by questionnaire how much they ran during pregnancy, when they stopped and at what stage in their pregnancy they gave birth – may also have led to inaccuracies.

Regular physical activity is important during pregnancy, but women should be guided by their prior level of fitness and what feels comfortable and appropriate.

Current NICE guidelines recommend that beginning or continuing a moderate course of exercise during pregnancy is not known to be associated with adverse outcomes. However, contact sports and vigorous or high-impact sports that may risk injury, falls or excessive joint stress are not recommended during pregnancy.

And although this study found that running during pregnancy wasn't associated with negative birth outcomes, it does not prove that any level of running is safe. It's advisable to consult your midwife or GP or first if you're considering an intensive running programme.

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